1. Field of the Invention
The present invention relates to an apparatus for tracing a centric relation of a mandible, and more particularly to a tracing apparatus in which a stylus used to check a centric relation of a mandible in the formation of prosthesis or artificial dentures is coupled to a nut-shaped ball supported by a base plate mounted to a dentition and a support plate threadedly coupled to the base plate in such a fashion that it can be selectively pivotable, thereby eliminating use of any separate dentition supporting wires.
2. Description of the Prior Art
Generally, a positional relation of the mandible with respect to the maxilla at a certain mandibular position is called a "maxillary-mandibular jaw relation". In order to make an accurate diagnosis or remedy for dental patients, it is frequently necessary to trace such a maxillary-mandibular jaw relation. In such a case, a three-dimensional maxillary-mandibular jaw relation, that is, longitudinal, lateral, and vertical maxillary-mandibular jaw relations, should be accurately traced. The reference point of a maxillary-mandibular jaw relation is simply called a "centric relation". This centric relation is a horizontal maxillary-mandibular jaw relation regularly and repeatedly traced in accordance with a patient's vertical dimension. Such a centric relation is a relaxed positional relation of the neuro-muscular system. There are various methods for tracing such a centric relation. One method is to instruct a patient to conduct longitudinal and lateral mandibular movements by himself under the condition in which tracing devices are mounted to the maxilla and mandible in the oral cavity of the patient, and to record movement paths respectively defining the longitudinal and lateral mandibular movements on a horizontal plane. Such movement paths are called "Gothic arch tracings". An example of such Gothic arch tracings is illustrated in FIG. 1. Referring to FIG. 1, a mandibular movement is defined by a forward longitudinal mandibular movement path 2-1 and opposite lateral mandibular movement paths 2-2 extending from an apex 2 on a recording or tracing plate 1. The reason why movement paths defining longitudinal and lateral mandibular movements are called "Gothic arch tracings" is because when a mandibular movement involving a forward longitudinal mandibular movement defined by the path 2-1 and opposite lateral mandibular movements defined by the paths 2-2 is figured on the tracing plate 1, the traced figure of the mandibular movement has a Gothic arch shape. The apex 2 is regarded as a starting point of the longitudinal and lateral mandibular movements. When a tracing stylus, which is attached to the mandible, as described hereinafter, is positioned at the apex 2, it is considered that a centric relation is established.
For a partially edentulous patient who is edentulous on the maxilla or mandible or a fully edentulous patient who is edentulous on both the maxilla and mandible, the centric relation of the mandible corresponds to the apex of a horizontal Gothic arch traced in accordance with an intrinsic vertical dimension of the patient. In this case, accordingly, a centric occlusion may be formed at the apex of the horizontal Gothic arch trace.
For a fully dentulous patient who is dentulous on both the maxilla and mandible, the horizontal centric relation of the mandible should be traced under the condition in which the maxillary and mandibular dentitions are spaced from each other to open the oral cavity, thereby preventing a stylus, which is used to record a Gothic arch trace of the patient, from coming into contact with a tooth of the patient during a tracing operation thereof. In other words, the tracing of the centric relation should be carried out with a vertical dimension elevated from an intrinsic vertical dimension of the patient.
In this case, however, the centric relation may vary in position on a tracing plate mounted to the maxilla due to a difference between the vertical dimension used and the intrinsic vertical dimension. In order to eliminate such a positional variation of the centric relation, the stylus should have a certain forward inclination corresponding to an angle of hinge rotation along the jaw movement path of the patient opening and closing the oral cavity while being in contact with the tracing plate in a state perpendicular to the tracing plate.
Meanwhile, if the position and orientation of the stylus mounted to the mandible is inaccurate with respect to the horizontal plane of the dentition, namely, the occlusal plane, that is, if the stylus is not in parallel to the occlusal plane, then it is in a laterally inclined state with respect to the maxilla. In such a state, it is impossible to expect an accurate centric relation tracing. Prior to a description associated with the mounted state of the stylus, the structure of the mandible will be described in conjunction with FIGS. 2a to 2e. FIG. 2a is a perspective view illustrating the structure of a mandible, FIG. 2b is a side view illustrating the maxilla denoted by the reference numeral 31, FIG. 2c is a bottom view of the maxilla 31, FIG. 2d is a side view illustrating an edentulous mandible, and FIG. 2e is a bottom view of the mandible shown in FIG. 2d. The mandible, which is denoted by the reference numeral 3, is hingably coupled to the maxilla 31 about a horizontal reference axis 3-1 (FIGS. 2a and 3). Teeth (dentition) 4 are fixed to the mandible 3. The horizontal reference axis 3-1 defines a Bonwill triangle 3-2, along with a central incisor edge between two mandibular central incisors. The Bonwill triangle 3-2 is indicated by a shaded portion in FIG. 2a. The horizontal plane of the dentition 4 defines an occlusal plane 3-3. A vertical plane extending vertically through the front central portion of the dentition 4 is called a "mid-sagittal plane". This mid-sagittal plane is denoted by the reference numeral 3-4. A stylus 5 is mounted at a cross point 3-5 between the center of opposite mandibular first molars and the mid-sagittal plane 34.
Typically, the mandible 3 has a structure shown in FIG. 3. As shown in FIG. 3, the Bonwill triangle 3-2 , which is a triangle extending from the horizontal reference axis 3-1 of the mandible 3 to the front central point of the dentition 4, defines a certain angle 3-6 along with the horizontal plane of the dentition 4, namely, the occlusal plane 3-3. This angle is called a "Balkwill angle".
For dentulous patients, the stylus 5 is mounted in such a fashion that its tip is positioned at the cross point 3-5 shown in FIG. 2a while being forwardly inclined with reference to a vertical plane orthogonal to the Bonwill triangle 3-2 by an angle corresponding to the Balkwill angle 3-6, as shown in FIG. 4. Generally, the stylus 5 is mounted in such a fashion that the inclination angle thereof, namely, the angle defined by the stylus 5 forwardly inclined from a line perpendicular to the occlusal plane 3-3 of the mandible, is 17.degree. in average. Theoretically, it is desirable for the stylus 5 to be inclined by an angle corresponding to the Balkwill angle 3-6. For some patients who do not conduct a regular mandibular hinge movement about the horizontal reference axis 3-1, however, it is necessary to adjust the angle of the stylus 5 without limiting it to 17.degree. in order to allow the stylus 5 to be positioned on a patient's hinge movement path, for example, at a point B1 or B2. Referring to FIG. 5, the stylus 5 is mounted to the dentition 4 of the mandible. As shown in FIG. 6, the stylus 5 is threadedly coupled to a support plate 6. A pair of wing plates 7 are threadedly coupled to opposite lateral ends of the support plate 6 by means of screws 8, respectively. The wing plates 7 are also fixedly mounted to opposite portions of the dentition on the mandible by means of coupling members 9 fixed to the wing plates 7 and wires 10 coupled to the coupling members 9, respectively. As shown in FIG. 6, the stylus 5 includes a bolt body 5-4 threadedly coupled to the support plate 6, and a nut body 5-21 threadedly coupled to the upper end of the bolt body 5-4. A spring pin 5-2 is slidably mounted in the bolt body 5-4 in such a fashion that its tip 5-1 is vertically protruded through the nut body 5-21 coupled to the bolt body 5-4. The spring pin 5-2 is always urged by a spring 5-3 so that it slide upwardly. The stylus 5 having such a configuration is mounted in such a manner that its tip; namely, the tip 5-1, is positioned on an extension line between the opposite mandibular first molars (denoted by the reference numeral 4-1 in FIG. 9a) while being in an inclined state. In other words, the bolt body 5-4 of the stylus 5 is threadedly coupled to the support plate in a state inclined with respect to the support plate 6. Also, the nut body 5-2 of the stylus 5 is threadedly coupled to the bolt body 5-4 in a state inclined with respect to the support plate 6. For this reason, the support plate 6 should be provided with a guide groove 6-1 for receiving a portion of the nut body 5-2 of the stylus 5 further downwardly protruded than other portions of the nut body 5-2 during the inclined coupling of the nut body 5-2, as shown in FIG. 6. Due to the provision of the guide groove 6-1, the stylus 5 coupled to the support plate 6 has a coupling portion having a length corresponding to only a distance L1 at one side thereof. As a result, the stylus 5 exhibits a reduced coupling force to the support plate 6, so that it may be loose. In this case, errors may be generated in the tracing of a Gothic arch for obtaining an accurate centric relation of the mandible. In practical cases, a dental pattern of a patient is first copied. A dental cast is then formed using plaster, based on the copied dental pattern. Thereafter, the dental cast is mounted to a general articulator in such a fashion that it conforms to a dentition of the patient. A tracing device, which is denoted by the reference numeral 20 in FIG. 5, is then assembled to the dentition of the articulator. Subsequently, the nut body 5-21 is threadedly coupled to the bolt body 5-4, as shown in FIG. 6. A cap 11, which is provided at its upper surface with an upwardly protruded pin 11-1, is then mounted on the nut body 5-21, as shown in FIG. 7. A tracing plate 1 is laid on the cap 11 in such a fashion that it is supported by the pin 11-1. Thereafter, a binding material 1-1, which has a property of being set within several minutes, is then applied to a palatal surface portion of the articulator facing the cap 11. The maxilla 31 of the articulator is then depressed in such a fashion that the tracing plate 1 is settled in the binding material 1-1, as shown in FIG. 7. In this state, the articulator is kept for several minutes until the binding material 1-1 is set. After the setting of the binding material 1-1, the maxilla 31 of the articulator is separated from the mandible 3 (not shown in FIG. 7). As a result, the stylus 5 and cap 11 are separated from the tracing plate 1. Of course, the tracing plate 1 is maintained in a state attached to the maxilla 31. Thereafter, the cap 11 is separated from the stylus 5. The cap 11 and tracing plate 1 may have configurations shown in FIG. 8, respectively. Referring to FIG. 8, the tracing plate 1 has a pin receiving hole 1-2 for receiving the pin 11-1 of the cap 11 so that it can be coupled to the cap 11. The tracing plate 1 is also provided at its front rear ends with coupling protrusions 1-3 formed integrally with the tracing plate 1. The coupling protrusions 1-3 have a construction upwardly bent from the horizontal tracing plate 1 so that they have an increased area contacting the binding material 1-1. Next, the tracing device 20 of FIG. 5 and the tracing plate 1 of FIG. 7, to which the set binding material 1-1 is integrally coupled, are completely separated from the articulator which is a dental model having maxillary and mandibular casts conforming to the maxilla and mandible of the patient. The separated tracing device 20 and tracing plate 1 are then mounted to the maxilla and mandible of the patient. Under this condition, the mandible of the patient moves longitudinally and laterally, thereby causing a Gothic arch trace to be formed on the tracing plate 1, as shown in FIG. 1. The tracing plate 1 recorded with the Gothic arch trace is then separated from the mouth of the patient, together with the set coupling material 1-1. A separate disc, which has a hole corresponding to the apex 2 of the Gothic arch trace, is subsequently attached to the tracing plate 1 in such a fashion that its hole is vertically aligned with the apex 2 of the Gothic arch trace. Thereafter, the tracing plate 1 attached with the disc and the tracing device 20 are fixed again to the maxillary and mandibular dentitions in the oral cavity of the patient. In this state, a bite material is put on desired portions of the maxilla and mandible between the tracing device 20 and tracing plate 1. After the bite material is set, the resulting structure including the maxillary and mandibular models and the set bite material is mounted again to the articulator in such a fashion that it has a centric relation conforming to the centric relation obtained in the oral cavity of the patient. Using this articulator, a dental prothesis is carried out while maintaining a stable centric occlusion. The above mentioned method is particularly useful for fully dentulous patients and partially dentulous patients. Even for patients exhibiting an unclear centric relation, in particular, patients who have used dentures for a lengthened period of time, an accurate centric relation can be obtained using the above mentioned method. However, the tracing device used in such a method may involve a problem in that the stylus may be pierced into the tongue of the patient due to an unstable positioning of the tracing plate. Furthermore, the tracing may be inaccurately carried out because the tracing plate has no lateral symmetry in a horizontal direction. In addition, it is difficult to accurately mount the tracing device to the articulator.
Conventional articulators for fully edentulous patients are disclosed in U.S. Pat. Nos. 4,273,533 and 4,279,595. Such articulators have a configuration including a tracing device adapted to be mounted to the maxilla, and a tracing plate adapted to be mounted to the mandible. However, such conventional articulators can be used only for edentulous patients. Furthermore, they have a limited applicability because they use a stylus constructed to extend vertically.